1. Field of the Invention
The present invention relates generally to the immune system and, more specifically, to a method for inducing antigen specific tolerance.
2. Description of Related Art
The host immune system provides a sophisticated defense mechanism which enables the recognition and elimination of foreign entities, such as infectious agents and neoplasms, from the body. When functioning properly, an effective immune system distinguishes between foreign invaders and the host's own tissues. The ability to specifically ignore the host's own tissues is called immune tolerance. Immune tolerance to self normally develops at birth when self antigens are brought to the thymus by antigen presenting cells (APCs). Thus, APCs play a crucial role in the "programming" of the immune system by specifically indicating which antigens are not to be considered foreign and, thereby, tolerated by the immune system. Autoimmune disease occurs when there is a breakdown in immune tolerance which results in a loss of tolerance to a particular antigen(s) and a subsequent attack by the host's immune system on the host's tissues which express the antigen(s). Immune tolerance to self is considered to be T cell dependent. Thus, the thymus plays a central role in the development of self tolerance. The antigen specificity of immune tolerance is determined by antigens presented by APCs. During the development of neonatal tolerance, these APCs migrate readily to the thymus and through their interaction with other cells of the immune system establish which antigens will be considered "self" and, thereby, will be sheltered from attack by the immune system. In the adult, the primary blood born antigen presenting cells are dendritic cells (DC), which localize at the corticomedullary junction of the thymus. Adoptive transfer of thymus explants has supported suggestions that these cells determine the antigen specificity of tolerance (Suzuki, et al., J. Immunol., 142:1463, 1989).
The ability of the immune system to distinguish between self and foreign antigens also plays a critical role in the area of tissue transplantation. The success of a transplant depends on preventing the immune system of the host recipient from recognizing the transplant as foreign and, in some cases, preventing the graff from recognizing the host tissues as foreign. For example, when a host receives a bone marrow transplant, the transplanted bone marrow may recognize the new host as foreign, resulting in graft versus host disease (GVHD). Consequently, the survival of the host depends on preventing both the rejection of the donor bone marrow as well as rejection of the host by the graff immune reaction.
At present, deleterious immune reactions are prevented or treated by general immune suppression in that the suppression is not antigen specific. Nonspecific immune suppression agents, such as steroids and antibodies to lymphocytes, put the host at increased risk for infection and development of tumors. In recent years, unwanted immune reactions have been prevented or treated with more selective immune suppression, such as with Cyclosporine A (CsA). CsA was thought to inhibit the proliferation of cytotoxic T cells while having relatively little effect on the proliferation of suppressor T cells. In addition, immunosuppressive therapy with CsA leads to depletion of the thymic medullary dendritic cells, the principal antigen presenting cells of the adult thymus. Although CsA has significantly improved the overall success of transplants and has shown some success with autoimmune diseases, it must be administered for the life of the patient. As a result, patients receiving such long-term CsA therapy are constantly at considerable risk for infections and neoplasms, as well as toxicity from the CsA.
Unwanted immune reactions which can result in autoimmune disease and transplant rejection can also be inhibited using steroids, azathioprine, anti-T cell antibodies, and more recently, monoclonal antibodies to T cell subpopulations. In addition to CsA, other selective immunosuppressive drugs that have been used include rapamycin, desoxyspergualine, and FK506. Unfortunately, when such agents are withdrawn, the unwanted immune reactions often recur. Ideally, it is a primary goal of transplantation immunology to achieve immune tolerance in an antigen-specific manner such that the unwanted immune reaction is prevented without inducing a state of generalized immune deficiency and accompanying increased susceptibility to disease. As a result, the tolerant host would remain capable of reacting appropriately to other antigens such as those associated with life-threatening infections or neoplasms.
Because of the major drawbacks associated with existing immunosupressive modalities, there is a need for a new approach for inducing immune tolerance in the host. The present invention provides such a novel method which allows antigen-specific tolerance while eliminating the need for protracted immunosuppressant therapy by utilizing a brief immunosuppressive treatment, just long enough to induce depletion of thymic medullary dendritic cells and allow recruitment of new APCs to the thymus. New antigen presenting cells containing the antigen to which specific tolerance is desired can be infused simultaneously or shortly thereafter. Since most individuals have only a limited capacity to regenerate the thymus, thymic regeneration can be stimulated by human growth hormone (hGH), human insulin-like growth factor-1 (hlGF-1), or related agents, after infusion of antigen presenting cells.
It has been known since 1967 that a connection exists between the anterior pituitary and the immune system, and specifically with growth hormone (GH). Two groups of investigators concluded from their studies that GH controls the growth of lymphold tissue (Pierpaoli and Sorkin, Nature, 215:834, 1967; Baroni, Experientia, 23:282, 1967). Subsequently, immunologic function was restored in the pituitary dwarf mouse by a combination of bovine somatotropic hormone and thyroxin (Baroni, et al., Immunol., 17:303-314, 1969.
In a sex-linked dwarf chicken strain, bovine GH (bGH) treatment resulted in enhanced antibody responses and bursal growth while thyroxine treatment stimulated thymus growth (Marsh, et al., Proc. Soc. Exp. Biol. Med., 175:351-360, 1984). However, neither treatment altered immune function in the autosomal dwarf chicken. Bovine GH therapy alone partially restored immunologic function in immunodeficient Weimaraner dogs (Roth, et al., Ann. J. Vet. Res., 45:1151-1155, 1984).
Studies have shown that mice with hereditary GH deficiency develop an impairment of the immune system associated with thymic atrophy, immunodeficiency, and wasting, resulting in a shortened life expectancy (Frabris, et al., Clin. Exp. Immunol., 9:209-225, 1971 ). It has been shown that an age-associated decline in the plasma concentration of thymulin (a thymic hormone) occurs and that plasma thymulin concentration increases in bGH-treated middle-aged and old dogs (Goff, et al., Clin. Exp. Immunol., 68:580-587, 1987). The authors suggest that exogenous GH may be useful for restoring some immune functions in aged individuals. Further, administration of hGH to C.sub.57 /B1/6J mice was found to reverse the inhibitory effect of prednisolone on thymus and spleen cellularity and on natural killer activity; administration of hGH without prednisolone had no effect, although at higher doses it induced a decrease of thymic parameters and natural killer activity with no effect on spleen cellularity, and relative weights (Franco, et al., Acta Endocrinologica, 123:339-344, 1990).
It has also been shown that GH induces T-cell proliferation in the thymus (Murphy, et al., FASEB Meeting Abstract, Atlanta, April 1991; Durum, et al., FASEB Meeting Abstract, Atlanta, April 1991). For recent reviews on the immune effects of GH, see Kelley, "Growth Hormone in Immunobiology," in Psychoneuroimmunology II, 2nd Ed., B. Ader, et al., eds., Acad. Press 1990, and Ammann, "Growth Hormone and Immunity," in Human Growth Hormone Progress and Challenges, L. Underwood, ed., Marcel Dekker, Inc., New York (1988), pp. 243-253; Weigent and Blalock, Prog. NeuroEndocrinImmunology, 3:231-241 (1990). It has been reported that all major immune cell types, including T-cells, B-cells, natural killer (NK) cells and macrophages, can be activated or expanded by GH (Kelly, Biochem. Pharmacol., 3.8:705, 1989).
One report states that locally generated IGF-I mediates GH action on T-lymphocytes through the type 1 IGF receptor (Geffner, et al., J. Clin. Endocrin. and Metab., 71:464, 1990). Also, Franco, et al., on p. 343, speculate that some of the effects of hGH on the immune system occur via IGF-I. Timsit, et al. (73rd Annual Meeting, Endocrine Society, June 19-22, 1991, abstract 1296) report that hGH and IGF-I stimulate thymic hormone function.
In addition, there have been data published documenting the ability of cells of the immune system to produce IGF-I-like molecules. These include activated alveolar macrophages (Rom, et al., J. Clin. Invest., 82:1685, 1988), human B-lymphocytes transformed with Epstein-Barr virus (Merimee, et al., J. Clin. Endocrin. Metab., 69:978, 1989), spleen and thymus tissues through detection of mRNA for IGF-I (Murphy, et al., Endocrinology, 120:1279, 1987), and normal T-cells (Geffner, et al., supra).
Data have also been presented suggesting that IGF-I produced locally in tissues such as the thymus or inflammatory sites might affect the growth and function of IGF-I-receptor-bearing T-lymphocytes (Tapson, et al., J. Clin. Invest., 82:950-957, 1988).
A statistically significant increase in thymus and spleen weight of hypophyscentomized rats infused for 18 days with IGF-I was observed as compared to control or treatment with GH (Froesch, et al., Growth Hormone Basic and Clinical Aspects, eds. O. Isaksson, et al., p. 321-326, 1987). Also reported was an increased thymic tissue in young GH-deficient rats treated with IGF-I (Guler, et al., Proc. Natl. Acad. Sci. USA, 85:4889-4893, 1988)) and an increase in the spleen of dwarf rats (Skottner, et al., Endocrinology, supra). Others have shown repopulation of the atrophied thymus in diabetic rats using either IGF-I or insulin; however, when the rats were immunized with bovine serum albumin (BSA) and boosted, serum anti-BSA antibodies showed no effect of insulin or IGF-I on the antibody response despite large effects on thymic and splenic size (Binz, et al., Proc. Natl. Acad. Sci. (USA), 87:3690-3694, 1990). IGF-I was reported to stimulate lymphocyte proliferation (Johnson, et al., Endocrine Society 73rd Annual Meeting, abstract 1073, Jun. 19-22, 1991).
Furthermore, IGF-I was found to repopulate the bone marrow cavity with hematopoietic cells (Froesch, et al., supra), stimulate erythropoiesis in hypophysectomized rats (Kurtz, et al., Proc. Natl. Acad. Sci. (USA), 85:7825-7829, 1988), and enhance the maturation of morphologically recognizable granulocytic and erythroid progenitors in suspension cultures of marrow cells (Merchav, et al., J. Clin. Invest., 81:791, 1988).
At nanomolar concentrations, IGF-I is a growth-promoting factor for lymphocytes (Schimpff, et al., Acta Endocrnol., 102:21-25, 1983). B-cells, but not T-cells, have recently been shown to possess receptors for IGF-I (Stuart, et al., J. Clinical Endo. and Met., 72:1117-1122, 1991). Also, IGF-I, as a chemotactic factor for resting and activated T-cells, stimulates an increase in thymidine incorporation into resting and activated T-cells. Normal T-cell lines show augmentation of basal colony formation in response to IGF-I (Geffner, et al., supra). It is also stated on p. 955 of Tapson, et al. (J. Clin. Invest., 82:950-957, 1988) that IGF-I produced locally in tissues, such as the thymus or inflammatory sites, might affect the growth and function of IGF-I receptor-bearing T-lymphocytes. However, IGF-I is reported to suppress in a dose-dependent manner IL-2-induced proliferative responses and in vitro antibody 15 responses of splenocytes (Hunt and Eardley, J. Immunol., 136:3991-3999, 1986).